1. Field of the Invention
The invention relates to a glass having a high zirconium oxide content, and to uses thereof.
2. Description of the Related Art
Glasses having a high zirconium oxide content have mainly been described in connection with alkali-resistant glass fibers for concrete reinforcement. Compared with E-glass, a substantially alkali-free aluminoborosilicate glass, fibers made from known ZrO2-containing glasses do have higher alkali resistance, but, in particular, their resistance in cement over a long period is inadequate. The alkali resistance of concrete-reinforcing fibers is of importance and is therefore usually to the fore during glass development, since the cement sets under highly alkaline conditions (pH values up to about 12.5). Besides the alkali resistance, however, the other chemical resistance, in particular the hydrolytic resistance, is clearly also of importance for long-term use as a reinforcing agent in concrete since it improves the long-term resistance.
Glasses which exhibit high resistance to water, acids and caustic lyes are interesting for a wide variety of applications, for example for pharmaceutical packaging or for inspection windows in process tanks, in particular if they additionally have high heat resistance.
A feature for high heat resistance is a high glass transition temperature Tg. In glasses having a high Tg, experience has shown that the so-called compaction (shrinkage) is low. This is shrinkage of glass parts during temperature treatment below the Tg, a property which can only be determined with sufficient accuracy with great experimental complexity and is of importance, for example, for applications in which very strict standards are set for the shape fidelity of the glass parts, for example for applications in display technology.
A high Tg and thus high heat resistance of the glass is likewise important in thin-film photovoltaic technology, in particular in solar cells based on chalcopyrites, such as copper indium diselenide (CIS), but also alternative compound semiconductors, such as CdTe. In thin-film photovoltaic technology, higher coating temperatures are thus possible, enabling optimized application of thin films with improved material quality, which results in an increase in the efficiency, for example in a solar cell.
For optical applications, glasses having high negative anomalous partial dispersion in the blue spectral region (xcex94Pg,F) are extremely interesting for correction of image aberrations. A disadvantage of the glasses in this series that have been disclosed hitherto is that they either have large amounts of PbO, which is undesired from environmental points of view, and/or have poor chemical resistance or that large amounts of the very expensive raw materials Nb2O5 and in particular Ta2O5 have to be used for lead-free substitution products, which makes economical manufacture much more difficult. Lead-free glasses of this type are disclosed in DE-A 27 29 706.
A wide variety of specifications in the patent literature which describe alkali-resistant glasses having high ZrO2 contents are also already known, but these still have disadvantages.
DE-A 29 27 445 describes an alkali-resistant glass composition which at least comprises 8% by weight of R2O, namely 8-17% by weight of Na2O and 0-5% by weight of K2O. CZ 236 744 also describes glass fibers made from mineral raw materials which comprise at least 8% by weight of Na2O and/or K2O.
British Patent Specification GB 1,290,528 describes glass compositions for the production of glass fibers which comprise from 13 to 23 mol % of R2O.
Glasses having such a high alkali metal content, as also occur in European Patent Specification EP 0 446 064 B1, which describes glass fiber materials for components of exhaust systems for internal-combustion engines (13-18% by weight of Na2O+K2O) and as also exhibited by the commercially available Cemfil fibers having a composition V1 (see below), exhibit poor hydrolytic resistance.
The same applies to the glass fibers in accordance with DE 17 96 339 C3 based on a glass comprising 11% by weight of Na2O and 1% by weight of Li2O and to the glasses converted into fibers in DE 40 32 460 A1, comprising 10-15% by weight of Na2O and 0.1-2% by weight of K2O.
The Patent Specification DD 293 105 A5 describes a process for the production of highly alkali-resistant glass fibers and products produced therefrom, in which the glass melt to be spun, besides SiO2, R2O3, ZrO2, RO and R2O (K2O, Na2O and/or Li2O), also contains fluoride. This fluxing agent can only be omitted if Li2O is present. These glasses, with 8-14% by weight of R2O, likewise have a relatively high alkali metal content.
The glass compositions from German Laid-Open Specification DE-A 2 406 888, which likewise have a high alkali metal content (10-25% by weight of R2O), comprise up to 20% by weight of oxides of the rare earth metals, for example cerium oxide or also naturally occurring mixtures of these oxides.
Rare-earth metal oxides, to be precise together with TiO2 in an amount of 0.5-16% by weight, where the TiO2 content is at most 10% by weight of the glass, are also present in the glasses from German Laid-Open Specification DE 31 07 600 A1. They furthermore comprise 0.1-1% by weight of Cr2O3. An essential aspect here is that the chromium is substantially in the trivalent state.
German Laid-Open Specification DE-A 26 14 395 describes Al2O3-free glasses, which have to comprise 0.5-10% by weight of Cr2O3+SnO2 for their alkali resistance, components which have the following disadvantages: Cr2O3 only dissolves in the glass flux with difficulty, and problems can also occur on use of chromium salts due to xe2x80x9cchromium knotsxe2x80x9d. SnO2 is a good nucleating agent and therefore promotes crystallization. The glasses furthermore require 0.05-1% by weight of SO3 as melt assistant, which can result in interfering foam and blow-hole formation.
DE-A 30 09 953 describes glass fibers which, besides ZrO2, must contain ThO2. This component is necessary in order to achieve alkali resistance. Owing to its radio activity, however, it is desirable to be able to omit this component.
EP 0 500 325 Al discloses glass fibers containing 5-18 mol % of TiO2. Their resultant chemical resistance is achieved at the expense of very high susceptibility to crystallization, which is particularly disadvantageous with respect to the spinnability of the glass melt.
JP 62/13293 B2 describes glass compositions containing at least 5% by weight of B2O3 for the core glass and coating of glass fibers. ZrO2 is merely an optional component. Although these glasses have high water resistance, this cannot, however, be guaranteed over the entire composition range owing to the high B2O3 contents at the same time as relatively high alkali metal contents, since water-soluble alkali metal borate phases can easily form.
DE-A 2 323 932 describes glass fibers which contain both P2O5 and also B2O3 in addition to very high contents of ZrO2 (8-16 mol %). The alkali metal content can vary within a broad range (1.5-25 mol %). Although such a high ZrO2 content greatly increases the alkali resistance, P2O5 reduces it again however. In addition, the hydrolytic resistance cannot be adequate over the entire composition range.
GB 2 232 988 A describes ZrO2-containing glass fibers which are coated with a thermoplastic resin in order to improve their alkali resistance. Owing to this additional process step, fibers of this type can only be produced expensively and in a complex manner. Fiber materials which can be used are glass fibers from the SiO2xe2x80x94ZrO2xe2x80x94R2O system with a fairly large variation latitude of the components and with further merely optional components, since, owing to the coating, the corresponding properties of the glass lose importance.
It is an object of the invention to provide a glass which has not only high caustic lye resistance, but also high hydrolytic resistance and relatively good acid resistance and which has high heat resistance and good processing properties.
This object is achieved by the glass having a high zirconium oxide content which is described in the main claim.
The glass according to the invention comprises from 54 to 72% by weight of SiO2. At higher contents, the meltability would be impaired, while at lower contents, glass formation would be more difficult. At least 55% by weight are particularly preferred, at least 59% by weight are very particularly preferred.
Al2O3, present in amounts of from 0.5 to 7% by weight, preferably up to 6% by weight, in a similar manner to SiO2 serves as glass former and thus improves the glass formation and makes a significant contribution toward improving the chemical resistance. However, excessively high contents would, in particular in the case of ZrO2-rich and low-R2O compositions, result in an increased tendency toward crystallization.
An essential aspect for the high alkali resistance is the ZrO2 content of the glass. It is therefore at least 8% by weight. The maximum content is 20% by weight, since otherwise the devitrification tendency increases excessively. The occurrence of crystals would result in glass flaws. A content of between 8 and 18% by weight is preferred. A content of at least 10% by weight is particularly preferred. A content of at most 15% by weight is particularly preferred.
The ZrO2/Al2O3 weight ratio is preferably greater than 2.
The alkali metal oxide(s), especially Na2O (2- less than 8% by weight of Na2O, preferably 3- less than 8% by weight, particularly preferably up to 4% by weight, and 0-5% by weight of K2O, preferably 1-2% by weight, with 2- less than 8% by weight of Na2O+K2O, preferably 3- less than 8% by weight, particularly preferably 3- less than 6% by weight) serve(s) to improve the meltability, i.e the reduction in the viscosity, and enable the high ZrO2 contents, since they increase the solubility of the ZrO2 in the glass. However, if the alkali metal contents are too high, the hydrolytic resistance, in particular, but also, albeit to a lesser extent, the caustic lye resistance would be impaired. It is preferred for both Na2O and K2O to be present.
With increasing proportion of Al2O3, the ZrO2 drops indirectly; this can be countered within the framework given by the stated limits by the presence of the alkali metal oxides. It is therefore preferred for the Al2O3/Na2O weight ratio to be  less than 1.64, which corresponds to an Al2O3/Na2O molar ratio of  less than 1. It is particularly preferred for not only the Al2O3/Na2O ratio, but also the Al2O3/R2O molar ratio to be  less than 1.
B2O3 is an optional component and improves the meltability by reducing the viscosity. However, its content should remain restricted to less than 5% by weight, preferably to 4% by weight, since B2O3 impairs the alkali metal resistance and in particular the acid resistance.
Of the alkaline earth metal oxides, which are present in the glass to the extent of greater than 5% by weight and at most 24% by weight, CaO is present in an amount of 3-11% by weight, preferably 3-10% by weight, while MgO in an amount of 0-10% by weight, SrO in an amount of 0-8% by weight and BaO in an amount of 0-12% by weight are optional components.
The alkaline earth metal oxides reduce the melt viscosity, suppress crystallization and also contribute toward an improvement in the alkali resistance. BaO in particular reduces the tendency toward crystallization.
It is therefore preferred for BaO to be present in an amount of at least 0.1% by weight. If the alkaline earth metal oxide content were too low, the meltability and processing properties in these low-alkali glasses would be impaired excessively, and they could no longer be converted into fibers, and the ZrO2 solubility would be too low. At a content greater than the maximum content mentioned, the glasses would devitrify, and crystallization would likewise occur. A total content of alkaline earth metal oxides of less than 23% by weight is preferred.
The glass may furthermore comprise 0-5% by weight of La2O3, preferably 0-4% by weight, and 0-4% by weight of TiO2. Addition of La2O3 improves the meltability of the glass, broadens the glass formation range and increases the refractive index. La2O3 and TiO2 principally contribute toward an improvement in the hydrolytic and caustic lye resistance, with La2O3 being more effective than TiO2. Excessive contents of La2O3 and TiO2 reduce the acid resistance and result in crystallization.
It is therefore preferred that the sum of La2O3, TiO2 and ZrO2 is  greater than 8.4. It is particularly preferred that the said sum is  greater than 10.
The glass may furthermore comprise up to 2% by weight, preferably up to 1% by weight, of each of Fe2O3, MnO2 and CeO2, where the sum of these three components should also not exceed 2% by weight, preferably should not exceed 1% by weight. These compounds are the usual impurities in naturally occurring raw materials of the glass constituents. In particular on use of the glasses according to the invention for the production of fibers for concrete reinforcement and as substrate in photovoltaic technology, inexpensive raw materials are of importance. On use of the glasses for optical purposes, the requirements of the purity of the glasses and thus of the purity of the raw materials are generally significantly greater. Here, the said sum and in particular the Fe2O3 content are preferably below 0.005% by weight.
For fining, the glasses may comprise conventional fining agents in conventional amounts, thus, for example, arsenic oxide, antimony oxide, chlorides or also fluorides, for example in each case as Ca or Ba halide, or, as preferred, SnO2.
Within the composition range of the main claim, there are two preferred composition ranges (in % by weight, based on oxide).
These are firstly:
SiO2 54-72, Al2O3 0.5-6, ZrO2 8-18, B2O3 0-4, Na2O 3- less than 8, K2O 0-5, with Na2O+K2O 3- less than 8, CaO 3-10, MgO 0-10, SrO 0-8, BaO 0.1-10, with CaO+MgO+SrO+BaO greater than 5xe2x88x92 less than 23, La2O3 0-5, TiO2 0-4.
The glasses of this composition range have high heat resistance. They have transition temperatures of at least 670xc2x0 C.
A further preferred composition range is the following:
SiO2 59-72, Al2O3 0.5-6, ZrO2 8-15, B2O3 0-4, Na2O 2-4, K2O 1-2, with Na2O+K2O 3- less than 6, CaO 3-10, MgO 0-10, SrO 0-8, BaO 0.1-10, with CaO+MgO+SrO+BaO greater than 5- less than 23, La2O3 0-5, TiO2 0-4.
This composition range includes glasses having coefficients of thermal expansion xc2x720/300 of from 4.5 to 6.0*10xe2x88x926/K.